The present invention relates to a stress measuring system capable of measuring stress and strain, and more specifically ,to a stress measuring apparatus capable of accurately measuring stress and strain from a remote place through light transmission lines such as optical fibers and the like.
Strain and stress acting on concrete, steel and the like as an object to be measured are measured by a method of directly applying a sensor unit on the object to be measured and a method of using a sensor unit contained in a case. Stress measuring apparatus using these methods employ as the sensing section thereof a strain gauge system, a Carlson system, a differential transformer system, an eddy current system, an optical fiber system, an oscillation string system or the like.
Any of the strain gauge system, the Carlson system, the differential transformer system and the eddy current system is an electric system which converts stress and strain into a minute electric signal by using a physical quantity as a medium.
Further, the optical fiber system makes use of the phenomenon that the light dispersion characteristics of an optical fiber change on receiving stress. Thus, the optical fiber system converts stress into a chance of a quantity of light and measures stress based on the change of the quantity of light.
Further, the oscillation string system is a system which uses a steel wire as an oscillation string and measures stress and strain by determining a tension of the string from the measured number of oscillation and converting the same by making use of a certain relationship existing between a change in tension of the steel wire and the natural frequency of the string.
On the other hand when stress is to be remotely measured by disposing a sensor unit relating to these systems at a desired sensing section, a signal sensed by the sensor unit must be transmitted to a measuring device and further electric power consumed by the sensor unit must be secured. The sensed signal can be transmitted by a method of converting the sensed signal into a light pulse signal and transmitting the signal to the measuring device through an optical fiber. Further, to secure electric power, a method of supplying electric power from the measuring device to the sensor section unit through a power line is usually employed when it is taken into consideration that the sensor unit is buried into an object to be measured.
However, when a conventional electric type sensor unit is employed, since a displacement of an object to be measured and stress based on the displacement are converted into a minute electric signal, electric noise is liable to be mixed with the minute electric signal in locations having a strong electric field and magnetic field such as power plants, substations, factories, and regions where much electric power is used, and the like, thus an accurate measurement is difficult. To improve resistance against noise, countermeasures such as the shield of the sensor unit, a circuit for removing noise from to a sensor signal and the like must be taken. Further, to make measurement at a pinpoint accuracy, calibration must be accurately executed and compensation must be executed according to the length of a lead wire. Therefore, a problem arises in that the sensor unit is increased in size and made complex and a manufacturing cost is increased accordingly.
In particular, since the electric type sensor unit is used for measurement in the vicinity of a flammable material such as oil, gas and the like depending upon a degree of power to be handled thereby, the sensor needs an explosion-proof design against an electric spark and countermeasures of insulation for securing insulation against the exterior of all the electric circuits.
Further, the sensor unit used in the optical fiber system is disadvantageous in an environmental change because when a temperature of the location where the sensor unit is mounted is changed, the characteristics of the sensor are also changed. This is because a change of stress is sensed as a change of a quantity of light. To overcome this drawback, an accurate calibration mechanism must be provided with, for example, a measuring device, which leads to an increase in size and complexity of the apparatus and an increase in cost.
Further, in the case of transmitting a signal sensed by the sensor unit of the aforesaid respective systems to a remote measuring device through an optical fiber, when the sensed signal is transmitted as an optical signal representing a change in a quantity of light, the quantity of light is changed by the change in an environmental temperature in the midway of the transmission, thus an accurate measurement is difficult. On the other hand, when a signal sensed by a sensor unit is transmitted by being converted into an optical pulse signal through an optical fiber, a conversion circuit for converting the sensed signal into the optical pulse signal must be assembled to a sensor unit, although the optical pulse signal is highly resistive against a change in temperature and the like. Thus, when an environmental change of the location where the sensor unit is mounted is taken into consideration, a conversion circuit having a pinpoint accuracy and high shield performance is needed, which eventually leads to an increase in size and complexity of the apparatus and an increase in cost.
Furthermore, in the case of supplying electric power to be consumed by a sensor unit through a power line, when thunders, a strong electric field or a strong magnetic field occur in the environment in the midway of the installation of the power line, electric noise is liable to be invaded into the power line. Thus, there may be a possibility of occurrence of a disadvantage that the sensor unit is burnt out or malfunction is caused by noise energy.